An ultrasonic diagnostic apparatus 100 includes an sound probe 101, a cable 102 and the apparatus body 103 as shown in FIG. 11. The sound probe 101 is connected to an electric circuit in the apparatus body 103 through the cable 102 and makes an interconversion on an ultrasonic wave and an electrical signal. The apparatus body 103 performs various kinds of signal processing including transmission processing in which a high-voltage transmission pulse to drive the sound probe 101 is output through the cable 102, reception processing in which a received signal, generated by the sound probe 101 on receiving an ultrasonic wave, is amplified to produce a received beam, brightness modulation of the received signal, and detecting blood flow information from the received signal, and generates an image signal based on the results of these kinds of signal processing.
FIG. 12 schematically illustrates the internal structure of the sound probe 101, which generally includes an array of transducer elements 104 where a plurality of transducer elements 105 are arranged in one direction. Signal lines are connected to the respective transducer elements 105 in the array of transducer elements 104 and bundled together to form the cable 102.
By getting the timings to apply the transmission pulses to those transducer elements 105 in the array of transducer elements 104 controlled by the apparatus body 103, an ultrasonic transmission beam to be emitted from this sound probe 101 can have any intended shape. On the other hand, by delaying the received signals obtained from these transducer elements 105 and then adding them together, an intended reception beam can be formed. As a result, a beam can be formed highly flexibly. For example, if the resolution in azimuth and direction (which will be sometimes referred to herein as “directivity”) is sharpened by forming narrow transmission and reception beams, the image quality of the ultrasonic image can be improved.
In general, a transmission beam is formed in the direction in which the transducer elements 105 are arranged (which will be referred to herein as a “azimuth direction”) by the technique described above. On the other hand, a transmission beam is shaped by an acoustic lens in the direction that intersects at right angles with the direction in which the transducer elements 105 are arranged and the depth direction in which ultrasonic waves are transmitted and received. The former direction will be referred to herein as a “elevation direction”.
Next, it will be described with reference to FIG. 13 how to form a beam in the elevation direction. FIG. 13 illustrates a cross section of the sound probe 101 in the elevation direction (which will be referred to herein as a “short-axis cross section”). The transducer element 105 is one of the transducer elements included in the array of transducer elements 104. As shown in FIG. 13, a backing member 106 which attenuates the vibration of the transducer element 105 is arranged on the lower surface of the transducer element 105. On the upper surface of the transducer element 105, on the other hand, arranged is an acoustic matching layer 107 to reduce the difference in impedance between the transducer element 105 and the subject (not shown). And on the upper surface of the acoustic matching layer 107, arranged is an acoustic lens 108. For example, if an acoustic lens 108 which is made of a material having a low sound velocity with respect to an organism is used, then the acoustic lens 108 may have such a shape as having a convex surface in the ultrasonic wave transmitting direction. As a result, the size of the transmission beam as measured in the elevation direction can be reduced.
A transmission beam is ordinarily converged so as to be narrowed at a certain depth level. In that case, at a level which is either shallower or deeper than the level at which the beam converges, the beam blurs so much that the azimuth resolution deteriorates.
When a transmission beam is shaped in the azimuth direction, the depth level at which the transmission beam converges can be changed by controlling the timing to apply a transmission drive pulse to the array of transducer elements 104 as described above. As a result, in the azimuth direction, the level at which the transmission beam narrows can be set in a broad range in the depth direction.
In the elevation direction, on the other hand, the transmission beam is shaped by the acoustic lens 108. That is why the transmission beam can be converged at only a particular depth level to be determined by the shape of the acoustic lens 108, and it is difficult to converge the transmission beam at any other depth level.
Thus, to overcome this problem, someone proposed a two-dimensional array of transducer elements which can change the depth levels at which an ultrasonic wave converges in both the azimuth and elevation directions by arranging a plurality of transducer elements in the elevation direction, too, and by controlling the timings to apply the transmission drive pulses. However, an sound probe 101 with such a two-dimensional array of transducer elements should have a great many transducer elements 105. That is why so many signal lines should be connected to such a huge number of transducer elements 105 that the cable 102 becomes too thick to use in practice. In addition, since the transmission beam is shaped two-dimensionally, the size of the electronic circuit needs to be increased, too.
Patent Document No. 1 discloses an sound probe which can overcome these problems. As shown in FIG. 14, the sound probe disclosed in Patent Document No. 1 includes transducer elements 105a, 105b and 105c which are arranged in the elevation direction. In observing a deep region of the subject using this sound probe, an ultrasonic wave is transmitted at a large aperture by using all of these transducer elements 105a to 105c. On the other hand, in observing a shallow region of the subject, an ultrasonic wave is transmitted at a small aperture by using only the transducer element 105b. 
According to Patent Document No. 1, by using only the transducer element 105b, the transmission beam can be narrower than the conventional one and the resolution of the resultant tomographic image can be increased.